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><A
NAME="SPI-SPI-PREPARE"
></A
>SPI_prepare</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN57558"
></A
><H2
>Name</H2
>SPI_prepare&nbsp;--&nbsp;prepare a plan for a command, without executing it yet</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN57563"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>SPIPlanPtr SPI_prepare(const char * <TT
CLASS="PARAMETER"
>command</TT
>, int <TT
CLASS="PARAMETER"
>nargs</TT
>, Oid * <TT
CLASS="PARAMETER"
>argtypes</TT
>)</PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN57568"
></A
><H2
>Description</H2
><P
>   <CODE
CLASS="FUNCTION"
>SPI_prepare</CODE
> creates and returns an execution
   plan for the specified command, but doesn't execute the command.
   This function should only be called from a connected procedure.
  </P
><P
>   When the same or a similar command is to be executed repeatedly, it
   might be advantageous to perform the planning only once.
   <CODE
CLASS="FUNCTION"
>SPI_prepare</CODE
> converts a command string into an
   execution plan that can be executed repeatedly using
   <CODE
CLASS="FUNCTION"
>SPI_execute_plan</CODE
>.
  </P
><P
>   A prepared command can be generalized by writing parameters
   (<TT
CLASS="LITERAL"
>$1</TT
>, <TT
CLASS="LITERAL"
>$2</TT
>, etc.) in place of what would be
   constants in a normal command.  The actual values of the parameters
   are then specified when <CODE
CLASS="FUNCTION"
>SPI_execute_plan</CODE
> is called.
   This allows the prepared command to be used over a wider range of
   situations than would be possible without parameters.
  </P
><P
>   The plan returned by <CODE
CLASS="FUNCTION"
>SPI_prepare</CODE
> can be used
   only in the current invocation of the procedure, since
   <CODE
CLASS="FUNCTION"
>SPI_finish</CODE
> frees memory allocated for a plan.
   But a plan can be saved for longer using the function
   <CODE
CLASS="FUNCTION"
>SPI_saveplan</CODE
>.
  </P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN57583"
></A
><H2
>Arguments</H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>const char * <TT
CLASS="PARAMETER"
>command</TT
></TT
></DT
><DD
><P
>      command string
     </P
></DD
><DT
><TT
CLASS="LITERAL"
>int <TT
CLASS="PARAMETER"
>nargs</TT
></TT
></DT
><DD
><P
>      number of input parameters (<TT
CLASS="LITERAL"
>$1</TT
>, <TT
CLASS="LITERAL"
>$2</TT
>, etc.)
     </P
></DD
><DT
><TT
CLASS="LITERAL"
>Oid * <TT
CLASS="PARAMETER"
>argtypes</TT
></TT
></DT
><DD
><P
>      pointer to an array containing the <ACRONYM
CLASS="ACRONYM"
>OID</ACRONYM
>s of
      the data types of the parameters
     </P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN57607"
></A
><H2
>Return Value</H2
><P
>   <CODE
CLASS="FUNCTION"
>SPI_prepare</CODE
> returns a non-null pointer to an
   execution plan.  On error, <TT
CLASS="SYMBOL"
>NULL</TT
> will be returned,
   and <TT
CLASS="VARNAME"
>SPI_result</TT
> will be set to one of the same
   error codes used by <CODE
CLASS="FUNCTION"
>SPI_execute</CODE
>, except that
   it is set to <TT
CLASS="SYMBOL"
>SPI_ERROR_ARGUMENT</TT
> if
   <TT
CLASS="PARAMETER"
>command</TT
> is <TT
CLASS="SYMBOL"
>NULL</TT
>, or if
   <TT
CLASS="PARAMETER"
>nargs</TT
> is less than 0, or if <TT
CLASS="PARAMETER"
>nargs</TT
> is
   greater than 0 and <TT
CLASS="PARAMETER"
>argtypes</TT
> is <TT
CLASS="SYMBOL"
>NULL</TT
>.
  </P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN57621"
></A
><H2
>Notes</H2
><P
>   <TT
CLASS="TYPE"
>SPIPlanPtr</TT
> is declared as a pointer to an opaque struct type in
   <TT
CLASS="FILENAME"
>spi.h</TT
>.  It is unwise to try to access its contents
   directly, as that makes your code much more likely to break in
   future revisions of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>.
  </P
><P
>   There is a disadvantage to using parameters: since the planner does
   not know the values that will be supplied for the parameters, it
   might make worse planning choices than it would make for a normal
   command with all constants visible.
  </P
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